Two basic tenets of Catholic teaching are that 1) God revealed
himself in a progressive revelation that was completed with the
death of the last apostle and 2) since then the Church's
understanding of that complete revelation has deepened and
developed.

Perhaps the classic model for understanding this process is
seen in the revelation given by God concerning His own Triune
nature. Certain critics of the Catholic Faith speak of the
doctrine of the Trinity as an "invention" of the
Church. However, it is closer to the mark to say that this truth
was discovered rather than invented. For the Church, so
far from creating anything, simply followed the clues left by God
in His complete revelation given through Scripture and Tradition.

The clues were essentially as follows:

There is but one God. This is the theme drummed into Israel by
the tradition and Scripture of both the Law and the Prophets.
"Hear O Israel! The Lord your God, the Lord is One!"
(Dt 6:4) is the very heart and soul of the Old Testament. Alone
among all the ancient nations of the earth, Israel is chosen by
God to be the one nation in covenant with this one God. Alone
among the nations of the earth, Israel is held to fidelity to
this one God through disaster, enslavement, deportation, conquest
and political humiliation. That there is one God is the
unshakable revelation given to the Jews.

However, something else is given to the Jews: the promise of a
Messiah. What this Messiah will look like is up for grabs at
first: Healer, Bringer of Peace, Conquering Davidic King,
Suffering Servant. All these hints roil about in the mix--until
Jesus appears and unites them all in His person. But He does
something more than that. While repeating the refrain of all
ancient Israel that God is One, He also forgives sin, which
prompts the Pharisees (and his disciples) to ask "Who can
forgive sin but God alone?" (Mk 2:7) He calls himself the
Son of David, but also implies that he is "David's
Lord" (Mk 12:35-37). He names himself "I AM": the
very name of God (Ex 3:14; Jn 8:58). In short, He claims to be
the Lord of the Universe Who led Israel through wilderness, gave
the Law, and called the Prophets. He is also, by His own account,
somehow distinct from the One He calls "my Father, who is
greater than I" (Jn 14:28). Yet at the same time, He insists
"I and the Father are one" (Jn 10:30). Moreover, he
teaches that there is yet Another, a Paraclete, a Spirit of
Truth, who proceeds from the Father (Jn 14:16-17; 15:26). And He
commands that His disciples baptize in the name of the Father,
Son and Holy Spirit (Mt 28:19).

This is the basic problem set by the Christian revelation. The
faith of the Church is in one respect identical to Israel's:
There is but one God. But it also cognizant of further
revelation, summed up in Simon Peter's declaration: "You are
the Christ, the Son of the Living God" (Mt 16:16) And it is
further complicated by the fact that both Jesus and His apostles
teach that the Spirit (or Holy Spirit or Spirit of Jesus or
Spirit of Truth or Advocate) is also a personal being who
convicts of sin, gives graces, enlightens and guides the Church
into all truth.

How then does the Church piece together this mysterious
revelation? Very slowly, and with a conscious reliance on the
Spirit of Truth to, in fact, do what Christ promised and guide
the Church into all truth (Jn 16:13).

Various attempts to reconcile the data are proposed by various
early thinkers. Some have certain insights to the Truth (for
example, it is a second century Christian (Tertullian) who coins
the term "Trinity", yet also certain failings (nearly
always due to the fact that these early Christian thinkers try to
make certain data "fit" by suppressing or ignoring
other data). Thus, for instance, some put forward the notion that
God is one by denying that the three Persons are distinct. Others
attempt to emphasize the distinctness of the persons and wind up
advocating something that looks like polytheism. Still others
attempt to float a theory that the Old Testament God is bad while
Jesus is the Good God of the New Testament, come to rescue us
from the Bad One.

All of these theories are weighed and sifted in the Church for
nearly three hundred years and the Church continually states
definitions of what she does not believe till finally a
theory appears which seems (like the devil tempting Christ) to
explain all the biblical data and yet which strikes at the very
heart of the Church's Faith: Arianism.

Arius developed the novel notion that Jesus was a supernatural
created being. Christ was, said Arius, vastly superior to us (as
an angel is) but still created and not of one being with the
Father. Arius argued that various Scriptures (such as "I and
the Father are one") referred to the oneness of Jesus' will
with God, not the oneness His being. And since Jesus was
created, according to Arius, the logical consequence was that
worshipping Him as we worship God was, in fact, a sin. This
"simple" theory, while appearing to be faithful to the
Oneness of God, also completely destroyed the preaching of the
Church that Jesus was literally "God with us." If Arius
was to be believed, then it meant that Jesus' death, like the
death of any other mere creature, could neither save from sin,
nor bestow on us what the apostles had promised: a participation
in the divine nature (2 Peter 1:4). For even Jesus cannot give
what he does not have.

How did the Church respond? It assembled in Council first at
Nicaea and later at Constantinople. At these Councils, the Church
reasserted the traditional understanding of Scripture that God
was indeed one (as Arius insisted) but that this oneness was a
oneness of union between the Persons of the One Godhead,
not a oneness of isolation. The Councils reaffirmed not only that
the Word was with God (as Arius taught) but that the Word was
God (as John 1:1 taught). In so doing, the Church made a historic
step. They delineated, not merely what they did not
believe about the Godhead (as was hitherto the case in questions
on this matter), but what they did believe. They chose a
series of careful statements (God from God, Light from Light,
True God from True God, begotten, not made, one in being with the
Father) that summarized not only all the Church had rejected in
her thinking, but what she positively asserted in the face of the
various attempts to suppress portions of the biblical data in
favor of false "simplicity."

In short, nothing was invented by the Church with respect to
the Trinity. Rather, the Church sought to prevent a
"simplifying" invention by Arius and remain true to all
the biblical data, not just pieces of it that Arius liked.
Paradoxically, in fighting the invention, the Church discovered
a far deeper understanding of what she had always believed and
formulated it in Nicene Creed.